Cooling device including an electrocaloric composite

ABSTRACT

Cooling devices, heat pumps, and climate controlling devices employing an electrocaloric composite of high thermal conductivity and significant electrocaloric effect are disclosed. The electrocaloric composites include a combination of one or more EC-fluoropolymers and their blends with one or more electric-insulating fillers of high thermal conductivity.

TECHNICAL FIELD

The present disclosure is directed to cooling devices, including heat pumps, refrigerators, air conditioning, and climate control devices, employing high thermal conductivity electrocaloric composites. The composite comprises an electrocaloric polymer and filler that has the characteristics of electrical insulation and high thermal conductivity. Such composites exhibit a high thermal conductivity and a sufficient electrocaloric effect to act as refrigerants in cooling devices.

BACKGROUND

Most conventional air conditioners and refrigerators achieve cooling through a mechanical vapor compression cycle (VCC). These systems suffer from low efficiency and there does not appear to be any economically viable avenue to significantly improve the efficiency of these VCC systems. Further, air conditioning is a major contributor to electric utility peak loads. The peak load electricity production is generally characterized by high generation costs and is provided by relatively inefficient and peak-poor polluting plants. Peak loads are also a major factor contributing to poor grid reliability. A related problem with today's VCC cooling technology is the adverse environmental impact of the refrigerant gases employed. Even though the hydrofluorocarbon (HFC) refrigerants in the current cooling systems are much safer for the ozone layer than previously used chlorofluorocarbons (CFC) refrigerants, they remain strong greenhouse gases. These factors necessitate a search for new cooling technologies for air-conditioning and refrigeration that possess improved energy efficiency, low cost and are environmentally friendly.

Cooling devices based on the electrocaloric effect (ECE) have been considered as an alternative to conventional VCC conventional heat pumps. Polymeric materials that exhibit an electrocaloric effect have been disclosed for use in cooling devices. See, e.g., U.S. Patent Application Publication 2011/0016885; Gu et al. “Simulation of chip-size eletrocaloric refrigerator with high cooling-power density”, Applied Physics Letters, 2013:102:112901-5; and Gu et al., “A chip scale electrocaloric effect based cooling device”, Applied Physics Letters, 2013:102:122904-4. The electrocaloric (EC) effect is a reversible temperature change that occurs in a polar material upon application of an electric field. The EC effect is a result of direct coupling between the thermal properties (such as entropy) and electric properties (such as electric field and polarization) in a dielectric material. In this type of material, a change in the applied electric field induces a corresponding change in polarization, which in turn causes a change in the dipolar entropy S_(p) as measured by the isothermal entropy change ΔS in the dielectrics. If the field change is carried out in an adiabatic condition, the dielectric will experience an adiabatic temperature change ΔT. Recently, large electrocaloric effect has been discovered and developed in modified polar-fluoropolymers such as poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) (P(VDF-TrFE-CFE)) terpolymer and polymer blends. Such polar-fluoropolymers have also been used as composites that exhibit enhanced polarization response. See, e.g., Chu et al., “Large Enhancement in Polarization Response and Energy Density of Poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) by Interface Effect in Nanocomposites”, Appl. Phys. Lett. 91, 122909 (2007). However, the art does not appear to have recognized certain deficiencies with EC materials and improvements in such materials are still needed to make them practical for use in cooling devices.

SUMMARY OF THE DISCLOSURE

Advantages of the present disclosure include a cooling device comprising at least one high thermal conductivity electrocaloric composite. The high thermal conductivity electrocaloric composite (HiThCd EC composite) includes one or more electrocaloric polymers in combination with one or more fillers. The fillers are advantageously electrically insulating so as to avoid substantial interference with the operation of the electrocaloric effect but have high thermal conductivity to improve the performance of the EC composite in the cooling device.

These and other advantages are satisfied, in part, by a cooling device comprising at least one high thermal conductivity electrocaloric composite. The high thermal conductivity electrocaloric composite includes (1) one or more EC fluoropolymers, such as those made from vinylidene fluoride (VDF) based polymers which contain at least one additional fluoro-monomer including trifluoroethylene (TrFE), chlorofluoroethylene (CFE), chlorodifluoroethylene (CDFE), chlorotrifluoroethylene (CTFE), tetrafluoroethylene (TFE), hexafluoropropylene (HFP), hexafluoroethylene (HFE), vinylidene chloride (VDC), vinyl fluoride (VF); and (2) one or more fillers which have high thermal conductivity but electrical insulating characteristics. Examples of such fillers include inorganic or organic high thermal conductivity fillers of oxides (such as Al₂O₃, MgO), nitrides (such as Si₃N₄, Aluminum nitride AlN, boron nitride (BN)), silicon carbide (SiC), certain carbons such as diamond, polyethylene highly oriented fibers (PE), and other similar fillers which possess high thermal conductivity. The fillers of the present disclosure can be in the size and shape of nano-fillers such as nano-tubes, nano-fibers, and nano-sheets and micron-sized fibers (fibers whose diameter are one or more microns).

Embodiments of the present disclosure include wherein the one or more fillers have thermal conductivity higher than 10 W/mK, e.g., higher than 20, 30 W/mK; the filler volume fraction in the EC composite is less than about 20 volume percent, e.g., less than about 10 volume percent, but higher than about 0.1 volume percent. The EC composite can advantageously have an electric field induce temperature changes, in the adiabatic condition, of more than 5° C. and an isothermal entropy change of larger than 22 Jkg⁻¹K⁻¹ in the temperature range from 0° C. to 50° C., under an electric field not higher than 100 MV/m under certain embodiments.

Additional advantages of the present invention will become readily apparent to those skilled in this art from the following detailed description, wherein only the preferred embodiment of the invention is shown and described, simply by way of illustration of the best mode contemplated of carrying out the invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference is made to the attached drawings, wherein elements having the same reference numeral designations represent similar elements throughout and wherein:

FIG. 1 is a schematic illustration of an idealized thermodynamic refrigeration cycle (Carnot cycle) employing an electrocaloric material.

FIG. 2 is a schematic illustration of a thermodynamic refrigeration cycle employing an electrocaloric (EC) material which takes into consideration of the temperature gradient in the heat exchange between the EC material and hot end and between the EC material and cold end.

FIG. 3 is an illustration of the cooling cycles in FIGS. 1 and 2 in a device configuration to show the heat exchange process between the EC material acting as a refrigerant and a hot and/or cold load. The four processes are: (A) The temperature of the ECE material is increased as the electric field is applied on; (B) The ECE material is in thermal contact with the hot end (heat sink) to eject heat; (C) The temperature of the ECE material is decreased as the applied electric field is removed; (D) The ECE material is in thermal contact with the cold load to absorbed heat, thus cools the load.

FIGS. 4A and 4B show mixing models of a HiThCd EC composite of the present disclosure. FIG. 4A illustrates a HiThCd EC composite having an EC polymer to filler arrangement in parallel and FIG. 4B illustrates a HiThCd EC composite having an EC polymer to filler arrangement in series.

FIG. 5 illustrates modeling results for the thermal conductivity for the HiThCd EC composite arrangements parallel (crosses) and series (circles) shown in FIG. 4.

FIG. 6 is a chart showing the comparative results of the induced polarization of an EC composite comprised of EC polymer (a PVDF based terpolymer) with boron nitride nano-fillers (5 vol %) to that of the EC polymer alone.

FIG. 7 is a schematic of a HiThCd EC composite with high thermal conductivity micron-diameter fibers and EC polymer.

DETAILED DESCRIPTION OF THE DISCLOSURE

The present disclosure is directed to cooling devices, including but not limited to heat pumps, refrigerators, air conditioning, climate control systems, etc. that include a high thermal conductivity electrocaloric composite to remove heat during the operation of the device. Advantageously, the electrocaloric composite exhibits a significant temperature/entropy change upon the application and removal of electric field or voltage and a cyclic temperature change (i.e. an increase and decrease in temperature upon a cyclic voltage applied to the composite).

In addition to optimizing the electrocaloric effect (ECE) of the material, the present disclosure describes electrocaloric composites that have improved thermal conductivity. By advantageously improving the thermal conductivity of the composite, optimized device design, operation frequency, and cooling power can be implemented.

For example, a key component of a cooling device is the transportation of entropy from the cold end to the hot end. The objective is to transport entropy from one temperature level to another temperature level in a reversible manner, that is, to transport the entropy without generating any additional entropy in the process. This requires a substance whose entropy depends on properties other than temperature. In the cooling devices of present disclosure, this substance is the electrocaloric material, whose entropy can be changed by external electric fields.

All steady state converters must be cyclic since the entropy carrying substance is not consumed. FIG. 1 illustrates an ideal cooling cycle (Carnot cycle) which includes two adiabatic and two isothermal processes. In the figure, S is entropy, T is the temperature, E is applied electric field. The subscripts h and c refer to high and low temperatures. The arrows show the dipole ordered and less ordered states of EC material.

For the Carnot cycle, the heat absorbed from the cold source is Q_(c)=T_(c)(S_(c)−S_(h)) and the coefficient of performance, COP=Q_(c)/W (where W is the total external work in the cooling cycle) can be expressed as

COP=T _(c)/(T _(h) −T _(c))  Eq. (1)

For a typical ECE based cooling cycle, there must exist a temperature gradient between the EC material and the hot-sink or cold load.

FIG. 2 is a schematic illustration of a more practical thermodynamic refrigeration cycle employing an electrocaloric material. This figure shows the temperature gradient required for heat transfer between the EC material and the hot end (at C) and cold end (at A). A high thermal conductivity EC material will reduce the temperature gradient and hence improve the efficiency and cooling power. The process A-B occurs during polarization that the EC material becomes hot with its temperature increasing from T_(A) to T_(B). In the process B-C the heat in EC material is transferred to a hot-sink and eventually reaches the hot-sink temperature. Then the depolarization occurs in the process C-D and the EC material temperature drops to T_(D), and finally the EC material absorbs heat from a cold-source and reaches T_(A) in the process D-A.

The cooling cycles in FIGS. 1 and 2 are presented for illustration. There are other types of cooling cycles, which employ regenerative processes and can generate temperature changes T_(h)−T_(c) larger than the temperature changes in FIG. 1 and also much larger than the adiabatic temperature change ΔT of the EC material.

In these devices, the EC refrigerant (i.e., ECE (or EC, for simplicity) material acting as a refrigerant) will exchange heat with the environment and thermal loads (such as the hot and cold ends) in order to achieve cooling, refrigeration, pumping heat, and climate control. FIG. 3 is an illustration of the cooling cycles in FIGS. 1 and 2 in a device configuration to show the heat exchange process between the EC material and a hot and/or cold load. As shown in FIG. 3, heat sink (3010) has a temperature of T_(h) and cold load (3030) has a temperature of T_(c). The cooling device also includes EC material 3020 and low thermal conductivity enclosure 3040 separating heat sink 3010 at T_(h) and cold load 3030 at T_(c). Arrows indicate the heat flow direction. In the heat pumping process as illustrated in FIG. 3, the heat exchanges between the EC material (3020) and the hot (3010) end and between the EC material (3020) and cold (3030) load. The heat exchange occurs in the EC material within a distance δ, the thermal diffusion length, at the interfaces between the EC material and the hot or cold elements. Here δ is the thermal diffusion length, δ=√{square root over (2k/(cω))} where ω is the angular frequency (=2π×operation frequency) and k is the thermal conductivity, and c is the specific heat. For EC polymers, the thermal conductivity k is very small, e.g., k=0.2 W/mK to 0.25 W/mK, compared with metals, which typically have thermal conductivities of greater than 100 W/mK. Hence when operating the cooling device at 10 Hz (operation frequency=10 Hz), the thermal diffusion length is only 73 μm, which means to maintain a high COP of the EC cooling device, the EC material thickness should not be larger than 73 μm, is too thin for many practical devices. For thicker EC polymers in the cooling device, this means a low operation frequency. Since for each cooling cycle in FIGS. 1 and 2, the heat removed from the cold end is fixed Q_(c)=T_(c)(S_(h)−S_(c)), where (S_(h)−S_(c)) is the entropy change in the EC material induced electrically, the cooling power of a heat pump is directly proportional to the operation frequency.

W _(c) =fQ _(c)  Eq. (2)

By enhancing the thermal conductivity by one order of magnitude, the thermal diffusion length can be increased by a factor of more than 3 or the operation frequency of the device can be raised by 3 fold, which will lead to enhanced cooling power and cooling device efficiency because δ=√{square root over (2k/(cω))}.

The inventors of the present disclosure discovered that by adding high thermal conductivity fillers to EC polymers to form HiThCd EC composites, the composites could be produced with high thermal conductivity while not significantly adversely affecting (reducing) the EC response. That is, the EC response of the resulting HiThCd EC composite is comparable or even better than that of the EC polymer without the filler.

In one aspect of the present disclosure a HiThCd EC composite includes one or more EC fluoropolymers in combination with one or more fillers. In an embodiment of the present disclosure, the HiThCd EC composite including one or more fluoropolymers and one or more filler has an adiabatic temperature change of higher than 4° C. under 100 MV/m or lower electric field, e.g., greater than about 6, 8 or even an adiabatic temperature change of higher than 10° C. under 100 MV/m or lower electric field. The HiThCd EC composite including one or more fluoropolymers and one or more filler can also have an isothermal entropy change of larger than 22 Jkg⁻¹K⁻¹ in the temperature range from 10° C. to 60° C., e.g., from 0° C. to 50° C., under an electric field not higher than 100 MV/m. The HiThCd EC composite of the present disclosure can also have a dielectric breakdown field higher than 200 MV/m, preferably higher than 300 MV/m, and more preferably higher than 400 MV/m

Fluoropolymers that are useful for the HiThCd EC composites of the present disclosure preferably exhibit a significant electrocaloric effect, e.g. an adiabatic temperature change of higher than 4° C. under 100 MV/m or lower electric field, e.g., greater than about 6, 8 or even an adiabatic temperature change of higher than 10° C. under 100 MV/m or lower electric field. In an embodiment of the present disclosure, the EC fluoropolymers useful for the EC composite have a dielectric constant higher than 7, preferably higher than 8 or 9, at room temperature

The EC polymers useful for the present EC composite include but are not limited to EC polymers made from vinylidene fluoride (VDF) based polymers which contain at least one additional fluoro-monomer including trifluoroethylene (TrFE), chlorofluoroethylene (CFE), chlorodifluoroethylene (CDFE), chlorotrifluoroethylene (CTFE), tetrafluoroethylene (TFE), hexafluoropropylene (HFP), hexafluoroethylene (HFE), vinylidene chloride (VDC), vinyl fluoride (VF), etc. These polymers can be copolymers or terpolymers, for example. The HiThCd EC composite can include one (i.e., neat) EC polymer or a blend of EC polymers.

In an embodiment of the present disclosure, HiThCd EC composite includes one or more terpolymers of P(VDF_(1-x-y)-R¹ _(x)-R² _(y)), where R¹ is selected from the group consisting of TrFE and TFE, and R² is selected from the group consisting of CFE, CTFE, CDFE, HFP, HFE, VDC, VF, and mixtures thereof. The variable x is in the range 0.01 to 0.49, and y is in the range from 0.01 to 0.15. Preferred terpolymers include P(VDF_(1-x-y)-TrFE_(x)-CFE_(y)), P(VDF_(1-x-y)-TrFE_(x)-CTFE_(y)), P(VDF_(1-x-y)-TrFE_(x)-HFP_(y)), P(VDF_(1-x-y)-TFE_(x)-CTFE_(y)), and P(VDF_(1-x-y)-TFE_(x)-CFE_(y)) (0.01<y<0.15 and 0.10<x<0.49) which exhibit significant EC responses. Preferred terpolymers include polyvinylidene fluoride-trifluoroethylene-chlorofluoroethylene (P(VDF-TrFE-CFE)), polyvinylidene fluoride-tri fluoroethylene-chlorodifluoroethylene (P(VDF-TrFE-CDFE)), polyvinylidene fluoride-trifluoroethylene-chlorotrifluoroethylene (P(VDF-TrFE-CTFE)), polyvinylidene fluoride-trifluoroethylene-hexafluoropropylene (P(VDF-TrFE-HFP)), polyvinylidene fluoride-trifluoroethylene-tetrafluoroethylene (P(VDF-TrFE-TFE)), polyvinylidene fluoride-trifluoroethylene-vinylidene chloride P(VDF-TrFE-VDC), polyvinylidene fluoride-trifluoroethylene-vinyl fluoride P(VDF-TrFE-VF), polyvinylidene fluoride-trifluoroethylene-hexafluoroethylene P(VDF-TrFE-HFE), polyvinylidene fluoride-tetrafluoroethylene-chlorofluoroethylene (P(VDF-TFE-CFE)), polyvinylidene fluoride-tetrafluoroethylene-chlorodifluoroethylene (P(VDF-TFE-CDFE)), polyvinylidene fluoride-tetrafluoroethylene-chlorotrifluoroethylene (P(VDF-TFE-CTFE)), polyvinylidene fluoride-tetrafluoroethylene-hexafluoropropylene (P(VDF-TFE-HFP)), polyvinylidene fluoride-tetrafluoroethylene-hexafluoroethylene P(VDF-TFE-HFE), polyvinylidene fluoride-tetrafluoroethylene-vinylidene chloride P (VDF-TFE-VDC), polyvinylidene fluoride-tetrafluoroethylene-vinyl fluoride P(VDF-TFE-VF), etc.

In another embodiment of the present disclosure, HiThCd EC composite includes one or more copolymers of P(VDF_(z)-CTFE_(1-z)), P(VDF_(z)-CFE_(1-z)), P(VDF_(z)-TrFE_(1-z)), P(VDF_(z)-TFE_(1-z)), P(VF_(z)-CTFE_(1-z)), P(VF_(z)-CFE_(1-z)), P(VF_(z)-HFP_(1-z)), P(VF_(z)-TrFE_(1-z)), and P(VF_(z)-TFE_(1-z)), the variable _(z) is not limited but can range from _(z) of 0.7 to 0.98. Preferred copolymers include P(VDF-CFE), P(VDF-CTFE), P(VDF-TFE), P(VDF-TrFE), and mixture thereof.

The HiThCd EC composite of the present disclosure can also include high energy (>1 MeV) electron irradiated copolymers of P(VDF_(1-x)-TrFE_(x)) and P(VDF_(1-x)-TFE_(x)), (0.2<x<0.5) and/or P(VDF_(1-x)-CTFE_(x)) (x<0.08). It has been shown that irradiation increases cross-linking of such copolymers and turns such copolymers into relaxors and enhances the ECE.

In another embodiment, the HiThCd EC composite includes blends of EC polymers. Such blends can include, for example, one or more terpolymers, such as those referenced above, with one or more copolymers, such as those referenced above. A preferred terpolymer and copolymer blend includes (1) one or more terpolymers of P(VDF_(1-x-y)-TrFE_(x)-R³ _(y)) (0.03<y<0.09, 0.5>x>0.25), P(VDF_(1-x-y)-TFE_(x)-R³ _(y))(0.03<y<0.1, 0.1<x>0.3) or a mixture thereof, where R³ is CTFE or CFE or HFP, in combination with (2) one or more copolymers of P(VDF_(1-z)-TrFE_(z)) (0.5>z>0.25) or P(VDF_(1-z)-TFE_(z)) (0.25<z<0.5), P(VDF_(1-z)-CTFE_(z)) (z<0.1), P(VDF_(1-z)-HFP_(z)) (z<0.05) or a mixture thereof.

In a preferred embodiment, the HiThCd EC composite includes EC fluoropolymers of P(VDF_(1-x-y)-R¹ _(x)-R² _(y)), where R¹ is selected from the group consisting of TrFE and TFE, and R² is selected from the group consisting of CFE, CTFE, CDFE, HFP, HFE, VDC, VF, and mixtures thereof. The variable x is in the range 0.01 to 0.49, and y is in the range from 0.01 to 0.15. Preferred terpolymers include P(VDF_(1-x-y)-TrFE_(x)-CFE_(y)), P(VDF_(1-x-y)-TrFE_(x)-CTFE_(y)), P(VDF_(1-x-y)-TrFE_(x)-HFP_(y)), P(VDF_(1-x-y)-TFE_(x)-CTFE_(y)), and P(VDF_(1-x-y)-TFE_(x)-CFE_(y)) (0.01<y<0.15 and 0.10<x<0.49) which exhibit significant EC responses, and blends of terpolymer and copolymer blend includes (1) one or more terpolymers of P(VDF_(1-x-y)-TrFE_(x)-R³ _(y)) (0.03<y<0.09, 0.2<x<0.49), P(VDF_(1-x-y)-TFE_(x)-R³ _(y)) (0.03<y<0.1, 0.1<x<0.4) or a mixture thereof, where R³ is CTFE or CFE or HFP, in combination with (2) one or more copolymers of P(VDF_(1-z)-TrFE_(z)) (z<0.5) or P(VDF_(1-z)-TFE_(z)) (z<0.3), P(VDF_(1-z)-CTFE_(z)) (z<0.1), P(VDF_(1-z)-HFP_(z)) (z<0.05) or a mixture thereof.

The weight ratio of any fluoropolymer in a blend can range from about 70 to 97 weight % of one polymer over others in the blend based on the total weight of the EC fluoropolymers in the blend. For example, for the terpolymer and copolymer blends, the blends can range from 70 wt % of the terpolymer and 30 wt % copolymer to 97 wt % of terpolymer and 3 wt % copolymer. Preferably, the copolymer in any blend containing copolymers is less than 15 wt % based on the total weight of the terpolymer and copolymer in the blend. Preferably any blend used with the EC composite exhibits a temperature change (adiabatic temperature change) of more than 5° C., induced under an electric field of 100 MV/m or lower, e.g., greater than about 6, 8 or even an adiabatic temperature change of higher than about 9° C. under 100 MV/m or lower electric field.

The fillers that are useful with the HiThCd EC composites of the present disclosure include one or more fillers that thermally conducting but electrically insulating. For the HiThCd EC composite, electrically conductive fillers in the composites can be detrimental since these fillers will cause electrical conduction and also reduce the operation field due to lowered dielectric strength (<150 MV/m). Thus, one consideration in selecting a filler is that the filler should not significantly affect the electric insulation property of the EC polymer since any electric conduction loss will lower the dielectric strength (reliability) and cause heating of the EC composite which will reduce the efficiency and cooling power of the cooling devices. An electrically insulating filler as used herein means a filler that can maintain the dielectric breakdown field of the EC polymers. Such fillers have electrical resistivity higher than about 10⁶ ohm meter (Ωm), preferably higher than 10⁸ Ωm and even higher than 10⁹ Ωm.

Since the filler can enhance the performance of the EC composite in a cooling device due to thermal conductivity, it is preferred that the filler has high thermal conductivity, e.g., greater than 10 W/Km, preferably greater than about 20, 30 or 50 W/Km. In an embodiment of the present disclosure, it is preferred that the one or more fillers have very high thermal conductivity, e.g., greater than about 100 W/mK, 200, 300, and even greater than about 500 W/mK. Preferably, the one or more fillers in the HiThCd EC composite will act to enhance the thermal conductivity of the HiThCd EC composite to more than about 2 and up to about 20 times that compared to the same EC fluoropolymer without the filler while having a minimal effect on the EC response.

Such fillers can include one or more organic high thermal conductivity fillers or inorganic fillers, such as those of oxides (such as Al₂O₃, MgO), nitrides Si₃N₄, Aluminum nitride AlN, boron nitride (BN)), carbides (silicon carbide (SiC)), certain carbons such as diamond, polyethylene highly oriented fibers (PE), and other similar fillers which possess high thermal conductivity with low electrical conductivity and mixtures thereof.

In an embodiment of the present disclosure, the filler in the EC composite is higher than about 0.1 volume percent but no more than about 30% by volume, e.g., no more than about 20%, 10%, and 5% by volume.

The fillers of the present disclosure can be in the size and shape of nano-fillers such as nano-tubes, nano-fibers, and nano-sheets and micron-sized fibers (fibers whose diameter are one or more microns). Nano-sized fillers with high aspect ratios are preferred, e.g., a high aspect ratio is when the length vs. diameter for nano-tubes or the length vs. thickness for nano-sheets is greater than about 50 or greater than about 100. Further by using anisotropic fillers such as fillers in the shape of fibers and sheets, the thermal conductivity of the EC composite can also be enhanced anisotropically. For example a EC composite including one or more fillers in the shape of a fiber can be fabricated to align the fibers to enhance the thermal conductivity of the composite along the fiber length direction while do not affect the thermal conductivity of the composite in the direction perpendicular. In an embodiment of the present disclosure, the EC composite has a thermal conductivity along one direction higher than 1 W/mK, e.g., higher than 2, 4, 6 W/mK, in the temperature range from −20° C. to 70° C., e.g., −10° C. to 60° C. and from 0° C. to 50° C.

The HiThCd EC composites of the present disclosure can be fabricated by mixing one or more EC fluoropolymers with one or more high thermal conductivity fillers by a variety of processes, such as using a standard melt extrusion process. Alternatively, the EC composites can be fabricated using a solution casting method in which the EC polymers are dissolved in a solvent and high thermal conductivity fillers dispersed in a solvent. The two solutions are mixed with a proper ratio, determined by the desired composite composition and then cast on a substrate to form a composite film. In the solution casting method, the surfaces of the fillers can be modified to enhance the uniform dispersion of the fillers in the fluoropolymer matrix. The surface modification can use, for example, 3-phosphonopropionic acid. Additional fabrication methods can include aligning micron-diameter fibers of high thermal conductivity and casting an EC polymer or polymer solution onto the aligned fibers.

In one aspect of the present disclosure, the fillers can be arranged in the HiThCd EC composite randomly or orderly. The two basic models representing the upper bound and lower bound of the thermal conductivity of EC composites k_(c) are when (1) the EC polymer and filler are arranged in parallel (the EC polymer and high thermal conductivity filler are arranged in parallel along the thermal conduction path) and (2) when the EC polymer and filler are arranged in series (the EC polymer and high thermal conductivity filler are arranged in series along the thermal conduction path of k_(c)). The parallel and series arrangements are illustrated in FIG. 4. As shown in the figure, light regions (4010) represent EC polymer and dark regions (4020) represent filler having high thermal conductivity and low electrical conductivity. FIG. 4 a illustrates a HiThCd EC composite having an EC polymer to filler arrangement in parallel and FIG. 4 b illustrates a HiThCd EC composite having an EC polymer to filler arrangement in series. The thermal conductivity of the two models can be expressed by the following equations:

k _(c) =f _(p) k _(p) +f _(m) k _(m)  Eq. (3)

and

k _(c)=1/((f _(p) /k _(p))+(f _(m) /k _(m)))  Eq. (4)

Where k_(p) and k_(m) are the thermal conductivity of the high thermal conductivity particle fillers and polymer matrix, and f_(p) and f_(m) are the volume fraction of the two constituents. Equation (3) is the thermal conductivity for parallel composite model (FIG. 4A) and equation (4) is that for a series model (FIG. 4B).

Presented in FIG. 5 is the results based on Eqs. (3) and (4) for a hypothetic composite with an EC polymer (k_(m)=0.2 W/mK) and with a filler having a thermal conductivity of k_(p)=250 W/mK. As shown in FIG. 5, the composites with the parallel structure (morphology) will yield a high thermal conductivity. With even 5 vol % of high thermal conductivity fillers, the thermal conductivity of an EC composite can be 12.5 W/mK, which is significantly larger than the thermal conductivity of an EC polymer without such a filler, e.g., a difference of more than about 12.3 W/mK. However, the thermal conductivity of the composite in the series model is not improved significantly.

In the design presented in FIG. 4, the two models actually represent the thermal conductivity of a composite along two perpendicular directions. Thus the composite has anisotropic thermal conductivity. That is a high k along one direction and a smaller k along the two perpendicular directions (or a perpendicular plane). This is acceptable for many EC based cooling devices, for example, the cooling cycles in FIGS. 1 and 2.

In addition to an orderly arrangement of the HiThCd EC composite, it is believed that a randomly oriented EC polymer filler composite can also achieve very high thermal conductivity. This can be achieved by combining one or more EC polymers with one or more nano-fillers such as nano-tubes (boron nitride nano-tubes, or nano-sheets of these materials). Such nano-tubes have a large shape aspect ratio that can form a thermal percolation path.

Carbon nano-tubes and carbon fibers, because of their very high thermal conductivity (>1,000 W/mK), have been used and investigated widely to enhance the thermal conductivity of polymers. However, carbon nano-tubes and fibers cannot be used for enhancing the thermal conductivity of EC composites here because of their low electrical resistivity (<10⁵ Ωm).

There are several highly electric insulation materials which possess high thermal conductivity that can be used as fillers for the EC composites of the present disclosure. These fillers include, for example, diamond, Si₃N₄, Al₂O₃, Boron Nitride (BN), MgO, Al₂O₃, Aluminum nitride AlN, polyethylene highly oriented fibers (PE), and SiC.

Since these high thermal conductivity fillers do not possess ECE, the volume fraction of such fillers in the EC composite should preferably be low as, for example, below 10%, so that the ECE of the EC composite will not be reduced significantly. For example, in the case of a parallel arrangement of EC polymer and filler as shown in FIG. 4 a, the reduction of ECE will be 10% when the filler volume fraction is 10%. The electrocaloric effect (isothermal entropy change ΔS_(comp)) of the composite can be expressed as:

ΔS _(comp) =ΔS _(poly)(1−f)  Eq. (5)

where f is the volume fraction of the high thermal conductivity fillers and ΔS is for the EC polymer

Although it is counter intuitive, it was observed that if the high thermal conductivity fillers are of a nano-size, such as nano-fibers with fiber diameter below 100 nm, the mixing of the EC polymer with nano-fillers such as BN nano-fibers will in fact enhance the polarization response, i.e., enhanced ECE, if the volume fraction of the nano-filler is low (for example, below 10%). FIG. 6 shows an enhanced polarization response for an EC polymer, e.g., a terpolymer made from vinylidene fluoride-based polymer, with boron nitride nano-sheets at about 5 vol. % compared to the same polymer without the boron nitride nano-sheets. The electrocaloric response of the EC polymer is proportional to the polarization. Hence, if the volume fraction of the high thermal conductivity nano-fillers is low (<10 vol %), the composites can exhibit both higher thermal conductivity and high EC response compared with the neat EC polymer.

BN nano-tubes (BNNT) and nano-sheets (BNNS) have been developed in recent years. These materials are known to have a very high aspect ratio, e.g., the length vs. diameter for nano-tubes or the length vs. thickness for nano-sheets is greater than 1,000. The large aspect ratio of these BNNTs or BNNSs promotes the formation of thermally connected networks even when the volume fraction of the nano-fillers is not high. Thus, BNNTs and BNNSs, or in general, nanotubes and nano-rods of the high thermal conductivity fillers are advantageous compared with nano-fillers of near sphere or very low aspect ratio (<10).

In addition to, or in place of nano-sized fillers, the HiThCd EC composites of the present disclosure can also include micron-diameter fibers of high thermal conductivity as illustrated in FIG. 7. FIG. 7 is a schematic of an EC composite with high thermal conductivity micron-diameter fibers 7020 and EC polymer 7010. For example, when the fibers with diameter less than 30 microns (such as Al₂O₃, BN fibers) are used, the composites will exhibit a significantly enhanced thermal conductivity along the fiber direction (see FIG. 7). For these composites, the inclusion of these high thermal conductivity fibers will reduce the EC response. Consequently, the volume fraction of the high thermal conductivity fillers should be low. The reduction of the EC response, for example, the isothermal entropy change ΔS_(comp) will be reduced from the ΔS_(poly) as shown in equation (5).

On the other hand, in order to have an effective heat exchange between the high thermal fibers and EC polymer matrix, the separation between the two neighboring high thermal fibers should be less than the thermal diffusion length 6. For 10 Hz operation, the separation should be less than 73 μm. Assuming 5% volume of high thermal conductivity fillers (thermal conductivity of the composite is >10 W/mK as shown in FIG. 5), the diameter of the fibers should be 17 μm or less so that the separation between the micro-fibers is 73 μm or less. In general, fibers of smaller diameter will allow for a closer distance between the fibers and hence improving the thermal transport between the high thermal conductivity fillers and EC polymers.

In an embodiment of the present disclosure, EC composites include EC fluoropolymers with high thermal conductivity polymer fibers, such as highly oriented polyethylene fibers and other highly oriented polymer fibers. Based on the modeling studies shown in FIG. 5, it is expected that such EC composites will have very high thermal conductivity while maintenance of the electrocaloric effect.

Additional fillers that are useful for the HiThCd EC composites of the present disclosure include insulating nano-particles of ferroelectric ceramics (EC ceramics) such as BaTiO₃, Ba(Ti_(1-x)Zr_(x))O₃, Ba(Ti_(1-x)Sn_(x))O₃ (x<0.3), (Ba_(1-x)Sr_(x))(Ti_(1-y)Zr_(y))O₃ (x<0.3, y<0.3), (Ba_(1-x)Sr_(x))TiO₃ (x<0.3) (Ba_(1-x)Sr_(x))(Ti_(1-y)Sn_(y))O₃ (0.05<x<0.3, 0.01<Sn<0.15), SrBiTa₂O₉, (Ba_(0.3)Na_(0.7))(Ti0_(.3)Nb_(0.7))O₃, Na_(0.5)Bi_(0.5)TiO₃, (PbLa)(Zr_(1-x)Ti_(x))O₃ (0.4<x<0.6), and (Pb(MgNb)O₃)_(1-x)—(PbTiO₃)_(x) (x<0.4), and these EC ceramics with additives (<5 weight %). Although these fillers possess relatively low thermal conductivity k<10 W/mK, because these fillers also exhibit significant ECE, their addition to the HiThCd may enhance the ECE of the HiThCd EC composite.

Only the preferred embodiment of the present invention and examples of its versatility are shown and described in the present disclosure. It is to be understood that the present invention is capable of use in various other combinations and environments and is capable of changes or modifications within the scope of the inventive concept as expressed herein. Thus, for example, those skilled in the art will recognize, or be able to ascertain, using no more than routine experimentation, numerous equivalents to the specific substances, procedures and arrangements described herein. Such equivalents are considered to be within the scope of this invention, and are covered by the following claims. 

What is claimed is:
 1. A cooling device comprising a high thermal conductivity electrocaloric (EC) composite as the refrigerant, wherein the high thermal conductivity electrocaloric composite can cycle through a temperature increase and decrease.
 2. The device of claim 1, wherein the high thermal conductivity EC composite has a thermal conductivity, along one direction, higher than 0.5 W/mK in the temperature range from 0° C. to 50° C.
 3. The device of claim 1, wherein the high thermal conductivity EC composite has a thermal conductivity, along one direction, higher than 1 W/mK in the temperature range from 0° C. to 50° C.
 4. The device of claim 1, wherein the high thermal conductivity EC composite has a thermal conductivity, along one direction, higher than 1 W/mK in the temperature range from −20° C. to 70° C.
 5. The device of claim 1, wherein the high thermal conductivity EC composite comprises one or more EC fluoropolymers having a significant electrocaloric effect in combination with one or more fillers, wherein the one or more fillers are electrically insulating and have high thermal conductivity.
 6. The device of claim 5, wherein the one or more EC fluoropolymers include a fluoropolymer made from vinylidene fluoride (VDF) based polymers which contain at least one additional fluoro-monomer including trifluoroethylene (TrFE), chlorofluoroethylene (CFE), chlorodifluoroethylene (CDFE), chlorotrifluoroethylene (CTFE), tetrafluoroethylene (TFE), hexafluoropropylene (HFP), hexafluoroethylene (HFE), vinylidene chloride (VDC), vinyl fluoride (VF) or mixture thereof.
 7. The device of claim 5, wherein the high thermal conductivity EC composite includes a blend of EC fluoropolymers.
 8. The device of claim 7, wherein the blend of EC fluoropolymers is a blend of a fluoropolymer that is a terpolymer with a fluoropolymer that is a copolymer.
 9. The device of claim 5, wherein the filler includes one or more of inorganic or organic high thermal conductivity fillers of oxides, nitrides, silicon carbide, certain carbons, polyethylene highly oriented fibers (PE) or mixtures thereof.
 10. The device of claim 5, wherein the filler volume fraction in the high thermal conductivity EC composite is less than 10 volume percent but higher than 0.1 volume percent.
 11. The device of claim 5, wherein the one or more fillers have thermal conductivity higher than 30 W/mK.
 12. The device of claim 5, wherein the one or more fillers are in a shape of nano-tube, nano-fiber, or nano-sheet.
 13. The device of claim 5, wherein the high thermal conductivity EC composite has a thermal conductivity, along one direction, higher than 0.5 W/mK in the temperature range from 0° C. to 50° C.
 14. The device of claim 5, wherein the high thermal conductivity EC composite has a thermal conductivity, along one direction, higher than 1 W/mK in the temperature range from 0° C. to 50° C.
 15. The device of claim 5, wherein the high thermal conductivity EC composite has a thermal conductivity, along one direction, higher than 1 W/mK in the temperature range from −20° C. to 70° C.
 16. The device of claim 5, wherein the one or more fillers are in the shape of a fiber of diameter larger than 1 micron and aligned which enhance the thermal conductivity of the composite along the fiber length direction while do not affect the thermal conductivity of the composite in the direction perpendicular to the aligned filler fibers.
 17. The device of claim 16, wherein the high thermal conductivity EC composite has a thermal conductivity, along one direction, higher than 1 W/mK in the temperature range from 0° C. to 50° C.
 18. The device of claim 16, wherein the high thermal conductivity EC composite has a thermal conductivity, along one direction, larger than 2 W/mK in the temperature range from −10° C. to 60° C.
 19. The device of claim 16, wherein the high thermal conductivity EC composite has a thermal conductivity, along one direction, larger than 4 W/mK in the temperature range from −20° C. to 70° C.
 20. The device of claim 1, wherein the high thermal conductivity EC composite has an electric field induce temperature change of more than 5° C. and an isothermal entropy change of larger than 22 Jkg⁻¹K⁻¹ under an electric field not higher than 100 MV/m.
 21. The device of claim 1, wherein the high thermal conductivity EC composite has an electric field induce temperature changes, in the adiabatic condition, of more than 5° C. and an isothermal entropy change of larger than 22 Jkg⁻¹K⁻¹ in the temperature range from 0° C. to 50° C., under an electric field not higher than 100 MV/m.
 22. The device of claim 1, wherein the high thermal conductivity EC composite has an electric field induce temperature changes, in the adiabatic condition, of more than 5° C. and an isothermal entropy change of larger than 22 Jkg⁻¹K⁻¹ in the temperature range from −10° C. to 60° C., under an electric field not higher than 100 MV/m.
 23. The device of claim 1, wherein the high thermal conductivity EC composite has a dielectric breakdown field higher than 200 MV/m.
 24. The device of claim 1, wherein the high thermal conductivity EC composite comprises one or more EC fluoropolymers and one or more EC ceramics and one or more high thermal conductivity fillers. 